For total timing, you hold the engine at a steady 2500 - 3000 rpm to check the "total" timing with all of the vacuum advance and centrifical advance from the distributor kicked in.
Initial timing is the timing at idle.
340's usually like to run around 17° initial timing, and 34° - 36° total timing at 2500 - 3000 RPM.
You can use the idle screw to keep the engine speed steady while you check your readings and make your settings, then back it off when you are done.